Hence it must be said that the angels, even inasmuch as they are immaterial substances, exist in exceeding great number, far beyond all material multitude. This is what Dionysius says (Coel. Hier. xiv): "There are many blessed armies of the heavenly intelligences, surpassing the weak and limited reckoning of our material numbers." The reason whereof is this, because, since it is the perfection of the universe that God chiefly intends in the creation of things, the more perfect some things are, in so much greater an excess are they created by God. Now, as in bodies such excess is observed in regard to their magnitude, so in things incorporeal is it observed in regard to their multitude. We see, in fact, that incorruptible bodies, exceed corruptible bodies almost incomparably in magnitude; for the entire sphere of things active and passive is something very small in comparison with the heavenly bodies. Hence it is reasonable to conclude that the immaterial substances as it were incomparably exceed material substances as to multitude.
Reply Obj. 1: In the angels number is not that of discrete quantity, brought about by division of what is continuous, but that which is caused by distinction of forms; according as multitude is reckoned among the transcendentals, as was said above (Q. 30, A. 3; Q. 11).
Reply Obj. 2: From the angelic nature being the nighest unto God, it must needs have least of multitude in its composition, but not so as to be found in few subjects.
Reply Obj. 3: This is Aristotle's argument (Metaph. xii, text 44), and it would conclude necessarily if the separate substances were made for corporeal substances. For thus the immaterial substances would exist to no purpose, unless some movement from them were to appear in corporeal things. But it is not true that the immaterial substances exist on account of the corporeal, because the end is nobler than the means to the end. Hence Aristotle says (Metaph. xii, text 44) that this is not a necessary argument, but a probable one. He was forced to make use of this argument, since only through sensible things can we come to know intelligible ones.
Reply Obj. 4: This argument comes from the opinion of such as hold that matter is the cause of the distinction of things; but this was refuted above (Q. 47, A. 1). Accordingly, the multiplication of the angels is not to be taken according to matter, nor according to bodies, but according to the divine wisdom devising the various orders of immaterial substances. _______________________
FOURTH ARTICLE [I, Q. 50, Art. 4]
Whether the Angels Differ in Species?
Objection 1: It would seem that the angels do not differ in species. For since the "difference" is nobler than the 'genus,' all things which agree in what is noblest in them, agree likewise in their ultimate constitutive difference; and so they are the same according to species. But all angels agree in what is noblest in them—that is to say, in intellectuality. Therefore all the angels are of one species.
Obj. 2: Further, more and less do not change a species. But the angels seem to differ only from one another according to more and less—namely, as one is simpler than another, and of keener intellect. Therefore the angels do not differ specifically.
Obj. 3: Further, soul and angel are contra-distinguished mutually from each other. But all souls are of the one species. So therefore are the angels.